Method of treating surface of aluminum substrate to increase performance of offshore equipment

ABSTRACT

Disclosed is a method of treating the surface of an aluminum substrate, including (a) forming a porous oxide film on the surface of the aluminum substrate and (b) applying a corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon. The method of treating the surface of the aluminum substrate enables the formation of the porous oxide film on the surface of the aluminum substrate through surface treatment so that the applied corrosion inhibitor is partially absorbed into the porous oxide film, thus exhibiting superior corrosion resistance and anti-fouling effects of the metal substrate, compared to conventional surface treatment methods involving coating only with a corrosion inhibitor.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. KR 2015-0164257 filed on Nov. 23, 2015 and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which are incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of treating the surface of an aluminum substrate to increase the performance of offshore equipment.

2. Description of the Related Art

Generally, offshore equipment for use in fabricating ships and large marine structures is mainly manufactured using alloys and other metal elements. The use of non-iron metals is increasing these days in order to realize offshore equipment exhibiting high durability and lightweightness.

Since offshore equipment inevitably undergoes metal corrosion due to seawater deposition, the surface of the aluminum substrate therefor is treated using a variety of methods to prevent the corrosion thereof.

Metal corrosion means that the properties of metal change through chemical or electrochemical reaction of the metal with materials contained in the ambient environment. Typically, metal corrosion occurs due to the presence of electrolytes such as soil, fresh water or seawater, the presence of cathode-anode potential difference, or the presence of a conductor for connecting a cathode and an anode. Also, metal corrosion is classified into dry corrosion, in the absence of water, and wet corrosion, in the presence of water, and takes place while corrosion current flows from the anode to the cathode due to a partial potential difference depending on the material and environment. In order to prevent the corrosion of the metal substrate, at least one corrosion factor, such as an electrolyte, a potential difference or a conductor, has to be eliminated.

Since metal corrosion cannot be actually completely prevented, the metal is inhibited from corroding in a manner of alleviating the corrosion or by suppressing corrosion to some extent for a predetermined time. Known methods for preventing corrosion include a surface treatment process through coating of the surface of a metal substrate with a corrosion inhibitor and an electron-chemical protection process for allowing predetermined potential to flow to the metal substrate to thus induce a kind of battery reaction. Recently useful is a surface treatment process for treating the surface of the metal substrate through coating with a corrosion inhibitor or the like.

The corrosion inhibitor includes asphalt, wax, petroleum and lubricating base oil, and is applied on the surface of a metal substrate to form a coating layer, in order to achieve an effect of physically protecting the surface of the metal substrate using physical strength and a chemical protection effect for preventing corrosive material such as oxygen or water from contacting the surface of the metal substrate through the interfacial action of the corrosion inhibitor, such as adsorption, solubilization, neutralization, dispersion or water substitution.

However, the coating layer on the surface of the metal substrate, formed through the coating process with the corrosion inhibitor, is lost over time, and thus the surface of the metal substrate corrodes. Hence, the coating with the corrosion inhibitor has to be repeated in order to form the coating layer on the surface of the metal substrate.

For example, the metal substrate, including aluminum and an aluminum alloy widely useful as a non-iron metal in deep seawater offshore equipment, has been used by being coated with a corrosion inhibitor in order to prevent the corrosion thereof.

However, when the surface of the aluminum substrate is coated with the corrosion inhibitor as mentioned above, bondability between the aluminum substrate and the corrosion inhibitor may decrease, thus easily losing the corrosion inhibitor coating layer formed on the surface of the aluminum substrate, undesirably deteriorating corrosion resistance.

Therefore, research is required into techniques for surface treatment of an aluminum substrate to improve corrosion resistance and anti-fouling effects of offshore equipment exposed to seawater and marine conditions.

CITATION LIST Patent Literature

-   (Patent Document 01) Korean Patent No. 10-0477382 (Laid-open date:     Sep. 5, 2002) -   (Patent Document 02) Korean Patent No. 10-0968333 (Laid-open date:     Aug. 1, 2008) -   (Patent Document 03) Korean Patent Application Publication No.     10-2012-0007506 (Laid-open date: Jan. 20, 2012) -   (Patent Document 04) Korean Patent No. 10-1301210 (Laid-open date:     Oct. 10, 2012)

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind the above problems encountered in the related art, and an object of the present invention is to provide a method of treating the surface of an aluminum substrate in order to increase corrosion resistance and anti-fouling effects of offshore equipment.

In order to accomplish the above object, the present invention provides a method of treating a surface of an aluminum substrate, comprising: (a) forming a porous oxide film on the surface of the aluminum substrate; and (b) applying a corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.

Also, the aluminum substrate may include Al7075.

Also, (a) may be performed using an anodizing process or a plasma electrolytic oxidation process.

Also, the anodizing process may be performed at room temperature at a voltage of 30 to 100 V for 1 to 3 hr.

Also, in (a), the oxide film may be formed at a thickness of 10 to 20 μm on the surface of the aluminum substrate.

Also, the corrosion inhibitor may be an oil-type corrosion inhibitor.

Also, the corrosion inhibitor may further include a viscosity controller.

Also, (b) may be performed using any one process selected from among spray coating, screen printing, brushing, and dipping.

Also, (b) may further include homogenizing the surface of the aluminum substrate, after the applying the corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.

In addition, the present invention provides an aluminum substrate for offshore equipment, manufactured by the above method.

According to the present invention, the method of treating the surface of an aluminum substrate enables the formation of a porous oxide film on the surface of the aluminum substrate through surface treatment so that the applied corrosion inhibitor is partially absorbed into the porous oxide film, thus exhibiting superior corrosion resistance and anti-fouling effects of a metal substrate, compared to conventional surface treatment methods involving coating only with a corrosion inhibitor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1A is a field emission scanning electron microscope (FE-SEM) image illustrating the surface of the anodized aluminum substrate according to the present invention;

FIG. 1B is an enlarged FE-SEM image of FIG. 1A;

FIG. 2 is of actual images illustrating changes in the surface of the anodized aluminum substrate after dropping treatment of an oil-type corrosion inhibitor, a solvent dilution-type corrosion inhibitor, a semisolid-type corrosion inhibitor and water according to the present invention; and

FIG. 3 is a graph illustrating the corrosion susceptibility of the surface-treated aluminum substrates in the example and comparative example.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a detailed description will be given of the present invention.

The present invention addresses a method of treating the surface of an aluminum substrate, comprising: (a) forming a porous oxide film on the surface of the aluminum substrate and (b) applying a corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.

Specifically, (a) is a step of forming a porous oxide film on the surface of the aluminum substrate.

The aluminum substrate may be used without limitation so long as it has any composition typically used in the art. Preferably useful is Al7075, which is an aluminum alloy comprising aluminum, magnesium, copper or zinc to thus exhibit superior mechanical properties and high usefulness in offshore equipment.

As for the aluminum substrate, an aluminum substrate, which is desmutted through degreasing treatment using any commercially available aluminum degreasing agent, etching treatment, and surface treatment with an acid, may be used such that the porous oxide film is formed on the surface thereof.

The formation of the porous oxide film on the surface of the aluminum substrate may be implemented through anodizing treatment or plasma electrolytic oxidation.

For example, in order to form the porous oxide film on the surface of the aluminum substrate through an anodizing process, the surface of the aluminum substrate is oxidized due to oxygen generated at the anode under the condition that the aluminum substrate is used as the anode and electricity is allowed to flow in the electrolyte, thereby obtaining a dense yet porous alumina film having superior mechanical properties.

The plurality of pores, which are formed in the surface of the aluminum substrate, are dependent on the temperature and voltage in the anodizing process, and the average pore size increases in proportion to an increase in the temperature and voltage.

Also, the thickness of the oxide film, which is formed on the surface of the aluminum substrate, depends on the temperature and voltage in the anodizing process, and the film thickness increases in proportion to an increase in the temperature and voltage.

In the present step, the surface of the aluminum substrate may be anodized under various temperature and voltage conditions, thus forming the porous oxide film having an average pore size suitable for end use as the aluminum substrate and a thickness able to maintain sufficient strength.

In the present step, the oxide film is preferably formed to a thickness of 10 to 20 μm on the surface of the aluminum substrate, and is provided in the form of a porous oxide film having an average pore size of 30 to 100 nm, thereby facilitating the absorption of the corrosion inhibitor that is to be applied in the subsequent procedure.

To form the oxide film having the above thickness and size, the surface of the aluminum substrate is anodized at 10 to 30° C. at a voltage of 30 to 100 V for 1 to 3 hr, whereby the porous oxide film having a uniform pore size is formed on the surface of the aluminum substrate.

If the above temperature is lower than 10° C., the oxide film formed on the surface of the aluminum substrate is too thin. On the other hand, if the above temperature is higher than 30° C., the current density applied to the pores formed in the surface of the aluminum substrate is further increased, and thus the pores are continuously grown in a direction perpendicular to the surface of the aluminum substrate, undesirably increasing the thickness of the oxide film. Given the above processing temperature range, the anodizing process is preferably carried out.

If the voltage is less than 30 V, it is difficult to form the oxide film to a sufficient thickness. On the other hand, if the voltage exceeds 100 V, it is easy to dissolve tips of the pores, undesirably greatly increasing the thickness of the oxide film. Given the above processing voltage range, the anodizing process is preferably carried out.

More preferably, anodizing treatment is carried out at 25° C. at 40 V for 3 hr, whereby the porous oxide film having a uniform pore size is formed on the surface of the aluminum substrate.

Also, (b) is a step of applying the corrosion inhibitor on the surface of the aluminum substrate having the porous oxide film formed thereon.

In the present step, the corrosion inhibitor, particularly a non-aqueous corrosion inhibitor is applied on the surface of the aluminum substrate having the porous oxide film formed thereon and is thus absorbed through the pores of the porous oxide film, thereby forming a coating layer having improved corrosion resistance and anti-fouling efficiency.

The corrosion inhibitor may be an oil-type corrosion inhibitor that includes a petroleum-based solvent.

The oil-type corrosion inhibitor may be obtained in a manner in which an environmentally friendly metal salt having high corrosion resistance and heat resistance, such as silicon, silver, magnesium, vanadium, zirconium, titanium, or hafnium, is alkalized with sodium hydroxide, potassium hydroxide or a mixture thereof and then mixed with a petroleum-based solvent or organic synthetic oil, thereby maximizing the corrosion resistance and anti-fouling effects of the surface-treated aluminum substrate. Without being limited thereto, a variety of known lubricating corrosion inhibitors, such as NP-7, NP-8, NP-9 and NP-10, may be used.

The oil-type corrosion inhibitor preferably has a viscosity of 1 to 1000 cP. When the corrosion inhibitor having the viscosity within the above range is applied on the surface of the aluminum substrate, the corrosion inhibitor is provided in the form of a coating layer at a thickness of 10 μm or more on the surface of the aluminum substrate while being absorbed into the pores of the oxide film.

In the present step, the oil-type corrosion inhibitor is further mixed with a viscosity controller so as to adjust the degree of absorption of the corrosion inhibitor into the pores in the porous oxide film formed on the surface of the aluminum substrate, thereby controlling the absorption efficiency and thickness of the coating layer.

The corrosion inhibitor coating layer, which is formed while being absorbed into the micropores in the oxide film, may exhibit superior corrosion resistance and anti-fouling effects because the retention time of the corrosion inhibitor on the surface of the aluminum substrate is increased and thus corrosion resistance and breakdown potential are increased, compared to aluminum substrates obtained through conventional surface treatment methods using only a corrosion inhibitor coating process.

To this end, the surface of the aluminum substrate is coated with the corrosion inhibitor using a process such as spray coating, screen printing, brushing or dipping.

The present step may further comprise homogenizing the surface of the aluminum substrate, after applying the corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.

The homogenizing treatment is performed in a manner such that the coating layer is uniformly diffused in the oxide film using a variety of known metal surface heat-treatment processes on the surface of the aluminum substrate having the coating layer formed thereon, thereby forming the coating layer at a uniform thickness on the surface of the aluminum substrate.

As the corrosion inhibitor coating layer is formed while being absorbed into the micropores in the oxide film, the retention time thereof on the surface of the aluminum substrate is increased, thus increasing corrosion resistance and breakdown potential, ultimately exhibiting superior corrosion resistance and anti-fouling effects, compared to aluminum substrates obtained through conventional surface treatment methods using only a corrosion inhibitor coating process.

Therefore, the method of treating the surface of the aluminum substrate according to the present invention enables the corrosion resistance and anti-fouling effects of offshore equipment, which is constantly exposed to seawater and marine conditions, to be increased, thus enhancing the performance of offshore equipment utilized in ships or marine plants, whereby economical industrial effects can be expected.

A better understanding of the present invention may be obtained through the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.

EXAMPLE

An aluminum substrate sample (Al7075) having a size of 100 mm×100 mm with a thickness of 3 mm was prepared, and then desmutted using nitric acid.

The aluminum substrate was disposed as an anode in an electrolyte containing 0.3 M oxalic acid and sulfuric acid, and the aluminum substrate was subjected to anodizing treatment at 25° C. at a voltage of 40 V for 3 hr. The surface of the anodized aluminum substrate was observed using FE-SEM. The results are shown in FIGS. 1A and 1B.

As illustrated in FIGS. 1A and 1B, the anodized aluminum substrate was confirmed to have the porous oxide film layer formed thereon.

To analyze the properties of the corrosion inhibitor, each of a non-aqueous corrosion inhibitor (oil-type), NP-1 (solvent dilution-type), anti-corrosive petrolactam (semisolid-type) and water (H₂O) was dropped onto the anodized aluminum substrate. After 5 min, the respective surfaces of the aluminum substrates were observed. The results are shown in FIG. 2.

As illustrated in FIG. 2, the aluminum substrate onto which the oil-type corrosion inhibitor was dropped or the aluminum substrate onto which the water was dropped sufficiently absorbed the corrosion inhibitor, whereby the surface of the aluminum substrate was confirmed to be sufficiently coated with the oil-type corrosion inhibitor. On the other hand, in the case of the aluminum substrate onto which the solvent dilution-type corrosion inhibitor was dropped, a portion of the solvent dilution-type corrosion inhibitor was absorbed into the oxide film layer. In the case of the aluminum substrate onto which the semisolid-type corrosion inhibitor was dropped, the corrosion inhibitor was not significantly absorbed. Hence, the oil-type corrosion inhibitor manifested the greatest adsorption properties for coating of the aluminum substrate.

Based on the above results, a non-aqueous corrosion inhibitor was sprayed onto one surface of the anodized aluminum substrate using a spray coating process, thus forming the coating layer on the surface of the aluminum substrate, whereby the surface of the aluminum substrate was subjected to corrosion resistance and anti-fouling treatment.

Comparative Example

The surface of an aluminum substrate was subjected to corrosion resistance and anti-fouling treatment in the same manner as in the above example, with the exception that anodizing treatment was not performed.

Test Example Test of Corrosion Resistance

The corrosion susceptibility of each of the aluminum substrates, which were surface-treated using the methods of Example and Comparative Example, was measured using a known corrosion resistance measuring process (ASTM F2129). The results are shown in FIG. 3.

As illustrated in FIG. 3, for the aluminum substrate that was surface-treated using the method of Example, the oil-type corrosion inhibitor was applied while being absorbed into the oxide film on the surface of the aluminum substrate, thus inducing metal oxidation at a higher potential. Furthermore, the breakdown potential of the aluminum substrate was higher than that of the aluminum substrate that was surface-treated using the method of Comparative Example. Therefore, when the aluminum substrate having the oxide film formed using the method of Example was coated with the corrosion inhibitor, the corrosion resistance and anti-fouling efficiency of the aluminum substrate were increased.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims. 

What is claimed is:
 1. A method of treating a surface of an aluminum substrate, comprising: (a) forming a porous oxide film on the surface of the aluminum substrate; and (b) applying a corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.
 2. The method of claim 1, wherein the aluminum substrate comprises Al7075.
 3. The method of claim 1, wherein (a) is performed using an anodizing process or a plasma electrolytic oxidation process.
 4. The method of claim 3, wherein the anodizing process is performed at room temperature at a voltage of 30 to 100 V for 1 to 3 hr.
 5. The method of claim 1, wherein in (a), the oxide film is formed at a thickness of 10 to 20 μm on the surface of the aluminum substrate.
 6. The method of claim 1, wherein the corrosion inhibitor is an oil-type corrosion inhibitor.
 7. The method of claim 6, wherein the corrosion inhibitor further comprises a viscosity controller.
 8. The method of claim 1, wherein (b) is performed using any one process selected from among spray coating, screen printing, brushing, and dipping.
 9. The method of claim 1, wherein (b) further comprises homogenizing the surface of the aluminum substrate, after the applying the corrosion inhibitor on the surface of the aluminum substrate having the oxide film formed thereon.
 10. An aluminum substrate for offshore equipment, manufactured by the method of any one of claims 1 to
 9. 